Multiphase electric drives are known in which various modulation types are used. Use of pulse width modulation (PWM) in multiphase drives is very common. Multiphase drives of this type are controlled as a function of the particular requirements that are present, according to one of the following methods:                use of sinusoidal current control (sinusoidal commutation),        use of block current control (block energization),        use of block voltages (control with block voltages),        use of sinusoidal voltage control with superimposed zero voltages.        
In principle, the above-mentioned techniques may be used in electric machines having any arbitrary phase number. Electric machines having three phases are most common in practice. However, there are also electric machines having a different phase number, for example two, four, five, six, seven, or nine phases.
In addition, so-called start-stop systems are already known. These systems are used for stopping and restarting an internal combustion engine for the purpose of reducing the fuel consumption and the exhaust gas emissions.
A start-stop system developed by the present applicant operates on the basis of conventional starters. The particular starter is controlled by an electronic control unit, and with the aid of a pinion meshes with an annular gear provided on the flywheel.
Furthermore, it has previously been proposed to implement a belt-driven starter generator on the basis of a claw pole generator, using an additional electronic control unit. In starter generators of this type, the phases are often directly connected to the battery via electronic semiconductor switches without using a clock method such as PWM.
To be able to recuperate higher amounts of energy in the case of braking, systems having fairly high voltages are necessary. At higher voltages, clocking of the supply voltage in the converter is necessary for starting the internal combustion engine in order to limit the current in the machine to a predefined maximum value. A clocked converter requires a DC link to high-capacitance capacitors in order to smooth the alternating components in the intake current. In the output stages, the dimensioning of the DC links often determines the space requirements for the particular output stage.
For vehicles having voltages greater than 14 V, and 42 V, for example, use of a step-up converter is already known. It is thus possible to allow current from the generator to already be delivered before an off-load voltage of 42 V is reached.
A device and a method for controlling a generator having an associated voltage converter are known from German Published Patent Appln. No. 199 03 426 A1, the voltage converter operating as a step-up converter. The mentioned control takes place in different ways in at least two subranges which are defined as rotational speed ranges or as voltage ranges. A first, lower-level control device is used for controlling the voltage converter, which operates as a step-up converter. A second control device is used as a controller for the excitation current flowing through the excitation winding of the generator. The two control devices are connected to one another and exchange information.
The lower-level control for the step-up converter is usually achieved with the aid of centered PWM control (center-aligned control), in which the control ratio is continuously increased. This variation of the control ratio is accompanied by a high DC link current. In particular for automotive applications, the presence of a high DC link current is critical, since the DC link capacitor is subjected to high ambient temperatures, and the mentioned ambient temperatures and the DC link current determine the service life of the DC link capacitor. A reduction in the DC link current opens the possibility of using lower-capacitance DC link capacitors, resulting in cost advantages. Another advantage lies in the possibility of increasing the maximum allowed ambient temperatures due to a reduction in the internal power loss of the DC link capacitor.